Dissecting the puzzle of tectonic lid regimes in terrestrial planets.

The surface tectonic style of rocky planets controls interior evolution, geologic activity, dynamo action, atmospheric composition, and thus, habitability. In the solar system, Earth is unique in exhibiting plate tectonics, but it also displays a protracted history of diverse tectonic regimes. To understand the dynamics of the coupled plate-mantle system, here we explore 2D hemispheric-scale thermochemical mantle convection models with self-consistent magmatism that reach the statistical steady state. By statistical analysis of model predictions, we quantitatively distinguish between six tectonic regimes, including the mobile, stagnant, sluggish, plutonic-squishy and episodic lids. In addition, we discover the episodic-squishy lid regime, characterized by alternating episodes of plutonic-squishy lid and mobile-lid behavior. By mapping out these regimes over a wide parameter space, we constrain the conditions for potential regime transitions during planetary cooling. Based on geological evidence of Earth's tectonic history, our results point to decreasing effective lithospheric strength during planetary evolution, consistent with previously-proposed physical weakening mechanisms. We also suggest an important role of the episodic-squishy lid for early Earth and present-day Venus. Thus, our study helps to understand the tectonic history of terrestrial planets as they cool over time.